This invention relates to compositions which can be used as capstock as well as in other applications. These compositions are especially useful for extruding into articles and for application to structural plastics such as poly(vinyl chloride) to prepare composites. The invention also extends to methods for manufacturing such extruded articles and to composites produced therefrom.
Poly(vinyl chloride) resin (hereafter xe2x80x9cPVCxe2x80x9d) has a combination of properties which make it particularly suitable for use as a structural material. In applications in which impact strength of the structural plastic is important, the PVC can be formulated with impact-modifier resins which improve the impact strength of the resulting composition. Such high impact-strength PVC compositions can be readily extruded or otherwise formed into a variety of articles which have excellent impact strength, toughness and other desired mechanical and chemical properties; for example as siding for buildings, shutters, technical profiles for window and door frames, rain carrying systems (e.g., gutters and downspouts), and fencings.
Such PVC compositions however have relatively poor weatherability characteristics, particularly poor color retention in darker grade colors such as browns and blues. The color is imparted to the PVC composition, for instance, by the use of colorants such as pigments or dyes, but exposure to sunlight causes unappealing changes in the colors. Such unappealing changes are more severe for darker than for light colors. Poor weatherability characteristics also causes reduction in impact strength leading to embrittlement and cracking and/or mechanical failure of the articles prepared from such compositions. Thus, there is a need for improving the weathering characteristics of such materials.
One remedy has been to incorporate stabilizing additives into the PVC composition, for example UV absorbers, thermal stabilizers and titanium dioxide. However, the resulting improvements to weatherability do not to meet the stricter industry-wide standards (Vinyl Siding Institute, January 1999, adopting ASTM D3679 performance specifications).
Another attempted remedy has been to apply another resinous material over the PVC to provide a surface that can withstand sunlight and other environmental conditions. Such a surfacing material is called xe2x80x9ccapstockxe2x80x9d. The capstock generally is much thinner than the substrate plastic, typically being about 10 to about 25% of the total thickness of the composite (i.e. the capstock and substrate plastic).
A suitable capstock material must possess a certain combination of processing properties and other physical, chemical, and aesthetic properties, including exceptional weathering characteristics such as excellent color retention and high impact strength. Moreover, the capstock also must not affect adversely those properties which make PVC such a widely used building material. In particular, the capstock compositions that are particularly aesthetically desirable do not have a shiny appearance but rather have a flat, or reduced gloss appearance.
Various types of polymer-based compositions have been disclosed for use as capstock, including PVC-based compositions and acrylic resin based compositions. A number of these polymer-based compositions are described in European Patent Application EP-A-473,379 which is incorporated herein by reference for its teaching of capstock compositions. This publication discloses a 20 capstock composition containing a blend of PVC resin and an acrylic copolymer. We have found, however, that the presence of PVC in capstock compositions such as those disclosed in EP-A-473,379 results in reduced weatherability. Moreover, the compositions according to this invention have a high gloss and therefore are not appropriate for certain applications (e.g., PVC siding and profile) in which a high gloss appearance is undesirable. Although this publication suggests that appearance can be controlled by the addition of flatting or matting agents, the addition of such components undesirably increases the cost and complexity of preparing capstock materials having reduced gloss.
Thus there is a need for a cost-effective, weatherable, capstock material having a high impact strength and adequate color retention, which also has a reduced, preferably low, gloss.
We have now discovered such a capstock composition that is capable of providing the requisite impact strength, high color retention, and reduced gloss without requiring additional PVC or flatting agents. This composition contains a particular combination of xe2x80x9chigh rubber corexe2x80x9d and xe2x80x9cmedium rubber corexe2x80x9d acrylic-based xe2x80x9ccore/shellxe2x80x9d polymers.
We have also discovered that processing the newly discovered capstock compositions in a plastics forming device (i.e., an extruder), wherein the metering and melt temperatures are carefully controlled, provides a capstock having the requisite impact strength, high color retention, and reduced gloss without the addition of PVC or flatting agents.
Therefore, an object of the present invention is to provide a capstock composition having a particular combination of xe2x80x9chigh rubber corexe2x80x9d and xe2x80x9cmedium rubber corexe2x80x9d acrylic-based xe2x80x9ccore/shellxe2x80x9d polymers capable of providing the requisite impact strength, high color retention, and reduced gloss without the addition of PVC or flatting agents.
A further object is to provide a process for preparing a capstock capable of providing the requisite impact strength, high color retention, and reduced gloss without the addition of PVC or flatting agents.
Another object of the invention is to provide a synthetic resin composite having a first layer of a reduced-gloss, weatherable, impact-resistant capstock composition and a second substrate layer of a thermoplastic resin.
These and other objects, as will become apparent from the following disclosure, are achieved by the present invention.
In the present invention, the problem of providing a weatherable, impact resistant capstock having low gloss is solved generally by providing a particular combination of xe2x80x9chigh rubber corexe2x80x9d and xe2x80x9cmedium rubber corexe2x80x9d acrylic-based xe2x80x9ccore/shellxe2x80x9d polymers having high impact strength, high color retention, and reduced gloss.
Thus, in a first aspect of the present invention is provided a capstock composition including:
(A) from 70 to 99 parts by weight of a first core/shell polymer including
(i) from 30 to 70 parts by weight of a core polymer including
(a) from 45 to 99.9 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer,
(b) from 0 to 50 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl acrylate monomer, and
(c) from 0.1 to 5 parts by weight of units derived from at least one crosslinker or graftlinker, and
(ii) from 30 to 70 parts by weight of a shell polymer grafted to the core polymer (A)(i) including
(a) from 80 to 99 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer, and
(b) from 1 to 20 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl methacrylate monomer; and
(B) from 1 to 30 parts by weight of a second core/shell polymer including
(i) from 70 to 92 parts by weight of a core polymer including
(a) from 50 to 99.9 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer,
(b) from 0 to 45 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl acrylate monomer, and
(c) from 0.1 to 5 parts by weight of units derived from at least one crosslinker or graftlinker; and
(ii) from 8 to 30 parts by weight of a shell polymer grafted to the core polymer (B)(i) including
(a) from 50 to 100 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer, and
(b) from 0 to 50 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl methacrylate monomer,
xe2x80x83wherein the shell polymer (B)(ii) has a shell molecular weight in the range of from 25,000 to 350,000 g/mol.
In a second aspect of the present invention, there is provided a capstock composition including
(A) from 75 to 85 parts by weight of a first core/shell polymer including
(i) from 35 to 65 parts by weight of a core polymer including
(a) from 95 to 99.5 parts by weight of units derived from n-butyl acrylate,
(b) from 0 to 4.5 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from n-butyl acrylate, and
(c) from 0.5 to 2.5 parts by weight of units derived from allyl methacrylate, and
(ii) from 55 to 65 parts by weight of a shell polymer grafted to the core polymer (A)(i) including
(a) from 90 to 99 parts by. weight of units derived from methyl methacrylate, and
(b) from 1 to 10 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer; and
(B) from 15 to 25 parts by weight of a second core/shell polymer including
(i) from 75 to 85 parts by weight of a core polymer including
(a) from 90 to 99.0 parts by weight of units derived from n-butyl acrylate,
(b) from 0.5 to 2.0 parts by weight of units derived from trimethylolpropane triacrylate, and
(c) from 0.0001 to 0.001 parts by weight of units derived from allyl methacrylate; and
(ii) from 15 to 25 parts by weight of a shell polymer grafted to the core polymer (B)(i) including
(a) from 95 to 100 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer, and
(b) from 0 to 5 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer,
xe2x80x83wherein the shell polymer (B)(ii) shell molecular weight is in the range of from 50,000 to 350,000 g/mol; and
(C) from 0.5 to 3.0 parts by weight of at least one UV light stabilizer.
In a third aspect of the present invention, there is provided a process for preparing a capstock capable of providing the requisite impact strength, high color retention, and reduced gloss including the steps of:
(I) preparing a mixture including
(A) from 70 to 99 parts by weight of a first core/shell polymer including
(i) from 30 to 70 parts by weight of a core polymer including
(a) from 45 to 99.9 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer,
(b) from 0 to 50 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl acrylate monomer, and
(c) from 0.1 to 5 parts by weight of units derived from at least one crosslinker or graftlinker, and
(ii) from 30 to 70 parts by weight of a shell polymer grafted to the core polymer (A)(i) including
(a) from 80 to 99 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer, and
(b) from 1 to 20 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl methacrylate monomer; and
(B) from 1 to 30 parts by weight of a second core/shell polymer including
(i) from 70 to 92 parts by weight of a core polymer including
(a) from 50 to 99.9 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer,
(b) from 0 to 45 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl acrylate monomer, and
(c) from 0.1 to 5 parts by weight of units derived from at least one crosslinker or graftlinker; and
(ii) from 8 to 30 parts by weight of a shell polymer grafted to the core polymer (B)(i) including
(a) from 50 to 100 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer, and
(b) from 0 to 50 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl methacrylate monomer,
xe2x80x83wherein the shell polymer (B)(ii) has a shell molecular weight in the range of from 25,000 to 350,000 g/mol;
(II) feeding the mixture into an extruder including a feed section and a metering section;
(III) metering and melting the mixture to form a melt, wherein the metering section has a temperature between 165xc2x0 C. and 190xc2x0 C., and the melt has a temperature between 165xc2x0 C. and 195xc2x0 C.;
(IV) forming the melt into a melt layer;
(V) extruding the melt layer; and
(VI) cooling the melt layer.
In a fourth aspect of the present invention, there is provided a synthetic resin composite including a substrate layer of an extrudable thermoplastic resin, and a capstock layer disposed thereon including a capstock composition of the first or second aspects of the present invention.
The term xe2x80x9creduced glossxe2x80x9d used herein refers to a surface having an average gloss value of 60 or less as measured with a 75 degree incident angle geometry glossmeter.
The term xe2x80x9cmedium rubberxe2x80x9d used herein denotes a core/shell polymer having from 30 to 70 weight percent of a rubbery polymer component, whereas the term xe2x80x9chigh rubberxe2x80x9d denotes a core/shell polymer having 70 or more weight percent of a rubbery polymer component.
The term xe2x80x9crubberyxe2x80x9d used herein denotes the thermodynamic state of a polymer above its glass transition temperature.
The term xe2x80x9cunits derived fromxe2x80x9d used herein refers to polymer molecules that are synthesized according to known polymerization techniques wherein a polymer contains xe2x80x9cunits derived fromxe2x80x9d its constituent monomers.
The term xe2x80x9cmolecular weightxe2x80x9d used herein refers to the weight average molecular weight of polymer molecules as determined by the gel permeation chromatography method.
The term xe2x80x9cshell molecular weightxe2x80x9d refers to the weight average molecular weight of the soluble polymer molecules which are obtained by dissolving an emulsion of the core/shell polymer particles in a suitable solvent (e.g., THF) and then separating (e.g., by filtration) the soluble polymer fraction from the insoluble polymer fraction. The weight average molecular weight of the soluble polymer is then determined by gel permeation chromatography to give the shell molecular weight.
The term xe2x80x9cgraftlinkerxe2x80x9d used herein refers to multi-functional monomers capable of forming multiple covalent bonds between polymer molecules of one type with polymer molecules of another type.
The term xe2x80x9ccrosslinkerxe2x80x9d used herein refers to multi-functional monomers capable of forming multiple covalent bonds between polymer molecules of the same type.
The term xe2x80x9calkyl (meth)acrylatexe2x80x9d used herein refers to both alkyl acrylate and alkyl methacrylate monomer compounds.
The term xe2x80x9cstagexe2x80x9d used herein is intended to encompass its broadest possible meaning, including the meaning conveyed in prior art such as U.S. Pat. Nos. 3,793,402, 3,971,835, 5,534,594, and 5,599,854, which offer various means for achieving xe2x80x9cstagedxe2x80x9d polymers.
The term xe2x80x9cpartsxe2x80x9d used herein is intended to mean xe2x80x9cparts by weightxe2x80x9d. Unless otherwise stated, xe2x80x9ctotal parts by weightxe2x80x9d do not necessarily add to 100.
The term xe2x80x9cweight percentxe2x80x9d used herein is intended to mean xe2x80x9cparts per hundredxe2x80x9d wherein the total parts add to 100.
It has now been found that a particular combination of a first xe2x80x9cmedium rubberxe2x80x9d acrylic-based core/shell polymer and a second xe2x80x9chigh rubberxe2x80x9d acrylic-based core/shell polymer is capable of being processed to provide a weatherable, impact resistant capstock having the requisite impact strength, high color retention, and reduced gloss appearance. The capstock compositions of the present invention have from 70 to 99, preferably from 75 to 95, and most preferably 75 to 85 parts by weight of a first xe2x80x9cmedium rubberxe2x80x9d core/shell polymer and from 1 to 30 parts, preferably from 5 to 30, and most preferably 15 to 25 parts by weight of a second xe2x80x9chigh rubberxe2x80x9d core/shell polymer.
The first xe2x80x9cmedium rubberxe2x80x9d core/shell polymers of the present invention can contain from 30 to 70, preferably from 35 to 60, and most preferably from 35 to 45 parts by weight of a rubbery core polymer and from 30 to 70, preferably 40 to 65, most preferably 55 to 65 parts by weight of a shell polymer grafted to the core polymer.
Such rubbery core polymers can contain from 45 to 99.9, preferably from 80 to 99.5, and most preferably from 94 to 99.5 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer, from 0 to 35, preferably from 0 to 20, most preferably from 0 to 4.5 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl acrylate monomer, and from 0.1 to 5, preferably from 0.5 to 2, most preferably from 0.5 to 1.5 parts by weight of units derived from at least one crosslinker or graftlinker.
Suitable C1-C8 alkyl acrylate monomers include, methyl-, ethyl-, propyl-, n-butyl, sec-butyl-, tert-butyl, pentyl-, hexyl-, heptyl-, n-octyl-, and 2-ethylhexyl-acrylate. N-butyl acrylate and ethyl acrylate monomers are preferred.
Suitable crosslinkers or graftlinkers include divinyl benzene, butylene glycol dimethacrylate, alkanepolyol-polyacrylates or alkanepolyol-polymethacrylates such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, butylene glycol diacrylate, oligoethylene glycol diacrylate, oligoethylene glycol dimethacrylate, trimethylolpropane diacrylate, trimethylolpropane dimethacrylate, trimethylolpropane triacrylate (xe2x80x9cTMPTAxe2x80x9d) or trimethylolpropane trimethacrylate, and unsaturated carboxylic acid allyl esters such as allyl acrylate, diallyl maleate, and preferably allyl methacrylate.
As long as the core polymer remains rubbery, the core polymer may also contain additional units derived from at least one ethylenically unsaturated copolymerizable monomer different from the C1-C8 alkyl acrylate monomers such as C1-C8 alkyl methacrylates, vinyl aromatic monomers, vinyl-unsaturated carboxylic acids monomers, and nitrogen-containing vinyl unsaturated monomers. Examples of the C1-C8 alkyl methacrylate monomers are ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, octyl methacrylate, and preferably methyl methacrylate. Examples of vinyl aromatic monomers include styrene, alpha-methyl styrene, para-methyl styrene, chlorostyrene, vinyl toluene, dibromostyrene, tribromostyrene, vinyl naphthalene, isopropenyl naphthalene, divinylbenzene and the like. The C1-C8 alkyl (meth)acrylate monomers are preferred in view of their enhanced weatherability of acrylic units over the other monomers. Examples of vinyl-unsaturated carboxylic acids monomers include methacrylic acid and acrylic acid. Examples of nitrogen-containing vinyl unsaturated monomers include acrylonitrile, methacrylonitrile, C1-C8 alkyl acrylamides, and C1-C8 alkyl methacrylamides.
The shell polymer grafted to the core polymer of the first xe2x80x9cmedium rubberxe2x80x9d core/shell polymers of the capstock composition of the present invention contain from 80 to 99, preferably from 85 to 97, and most preferably from 92 to 96 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer, and from 1 to 20, preferably from 3 to 15, most preferably from 4 to 8 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl methacrylate monomer.
Suitable polymers for the outer shell of the first core/shell polymer require that they have a glass transition temperature (xe2x80x9cTgxe2x80x9d) above 20xc2x0 C. and therefore may also contain one or more units derived from ethylenically unsaturated copolymerizable monomers which are different from the at least one C1-C8 alkyl methacrylate monomer.
Suitable ethylenically unsaturated copolymerizable monomers include one or more of any of the following monomers: C1-C8 alkyl (meth)acrylates, acrylonitrile, methacrylonitrile, divinyl benzene, alpha-methyl styrene, para-methyl styrene, chlorostyrene, vinyl toluene, dibromostyrene, tribromostyrene, vinyl naphthalene, isopropenyl naphthalene, as well as higher carbon (C9-C20) alkyl (meth)acrylates such as decyl acrylate lauryl methacrylate, lauryl acrylate, stearyl methacrylate, stearyl acrylate, isobornyl methacrylate, and the like. The C1-C8 alkyl (meth)acrylate monomers are preferred for their enhanced weatherability characteristics. Most preferred are the C1-C8 alkyl acrylate monomers.
The shell molecular weights of the shell polymer are in the range of from 10,000 to 1,000,000 and preferably in the range of from 50,000 to 500,000 g/mol. Controlling molecular weights in this range can be accomplished by one of various methods known in the art and is preferably accomplished by preparing the outer shell polymers in the presence of one or more chain transfer agents. Increasing the chain transfer agent amount lowers the shell molecular weight. The amount of chain transfer agent present can be in the range of from 0 to 5, and preferably from 0.001 to 1.0, weight percent based on shell polymer weight. These amounts are based on using common chain transfer agents such as the alkyl mercaptans; various chain transfer agents having lower chain transfer coefficients will require a concomitant increase in their amount in order to control shell molecular weight. Other ways of reducing molecular weight include increasing the polymerization temperature and/or with a gradual feeding of the monomers, increasing the initiator level, feeding in the shell monomers neat versus preemulsifying the monomers, and by controlling the degree of mixing of the reaction medium. If the amount of chain transfer agent is lower than 0.001 weight percent of the shell polymer weight then the shell molecular weight may not be well controlled leading to variability in the properties of the capstock composition, and requiring one or more other means of controlling shell molecular weight (i.e., increasing initiator level). If the amount of chain transfer agent is too high then the degree of grafting of the shell polymer is reduced.
Common chain transfer agents or mixtures thereof known in the art include the C4-C18 alkyl mercaptans, mercapto-group-containing acids, thiophenols, carbon tetrabromide, carbon tetrachloride, and the like. They may be used alone or as mixtures thereof. Examples of the C4-C18 alkyl mercaptans include butyl mercaptan, hexyl mercaptan, n-octyl mercaptan, decyl mercaptan, lauryl mercaptan. t-dodecyl mercaptan and n-dodecyl mercaptan are preferred. Numerous chain transfer agents and their coefficients for controlling molecular weight are found in The Polymer Handbook, 3rd Ed., Brandrup and Immergut, Eds., Wiley Interscience, 1989, pp. II/81-II/141, which is incorporated by reference to its disclosure of chain transfer agents and chain transfer constants.
The second xe2x80x9chigh rubberxe2x80x9d core/shell polymers of the present invention contain from 70 to 92, preferably from 72 to 88, and most preferably from 75 to 85 parts by weight of a rubbery core polymer and from 8 to 30, preferably from 12 to 28, and most preferably from 15 to 25 parts by weight of a shell polymer grafted to the core polymer.
Such rubbery core polymers contain from 50 to 99.9, preferably from 80 to 99.9, and most preferably from 90 to 99.9 parts by weight of units derived from at least one C1-C8 alkyl acrylate monomer, from 0 to 45, preferably from 0 to 15, and most preferably from 0 to 5 parts by weight of units derived from at least one ethylenically unsaturated copolymerizable monomer different from the at least one C1-C8 alkyl acrylate monomer, and from 0.1 to 5, preferably from 0.5 to 2, most preferably from 0.7 to 1.5 parts by weight of units derived from at least one crosslinker and graftlinker. It is preferred that the rubbery core polymers contain from 0.0001 to 0.1 parts by weight total of units derived from at least one crosslinker and at least one graftlinker.
Typical C1-C8 alkyl acrylate monomers include, methyl-, ethyl-, propyl-, n-butyl, sec-butyl-, tert-butyl, pentyl-, hexyl-, heptyl-, n-octyl-, and 2-ethylhexyl-acrylate. N-butyl acrylate and ethyl acrylate monomers are preferred. Suitable crosslinker or graftlinker monomers include divinyl benzene, butylene glycol dimethacrylate, alkanepolyol-polyacrylates or alkanepolyol-polymethacrylates such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, butylene glycol diacrylate, oligoethylene glycol diacrylate, oligoethylene glycol dimethacrylate, trimethylolpropane diacrylate, trimethylolpropane dimethacrylate, trimethylolpropane triacrylate or trimethylolpropane trimethacrylate, and unsaturated carboxylic acid allyl esters such as allyl acrylate, diallyl maleate, and allyl methacrylate. Although trimethylol propane triacrylate (xe2x80x9cTMPTAxe2x80x9d) is the preferred crosslinker and allyl methacrylate is the preferred graftlinker, both may be used interchangeably in the same core/shell polymer.
As long as the core polymer remains rubbery, the core polymer of the second xe2x80x9chigh rubberxe2x80x9d core/shell polymer may also contain additional units derived from at least one copolymerizable monomers such as C1-C8 alkyl (meth)acrylate, vinyl aromatic monomers such as styrene, vinyl-unsaturated carboxylic acids monomers such as methacrylic acid, and nitrogen-containing vinyl unsaturated monomers such as acrylonitrile. The C1-C8 alkyl (meth)acrylates are the preferred additional monomers in view of their superior weatherability.
The shell polymer grafted to the core polymer of the second xe2x80x9chigh rubberxe2x80x9d core/shell polymers of the present invention contains from 50 to 100, preferably from 90 to 100, and most preferably from 98 to 99.9 parts by weight of units derived from at least one C1-C8 alkyl methacrylate monomer. The shell molecular weight is in the range of from 25,000 to 350,000, preferably in the range of from 50,000 to 200,000, and most preferably in the range of from 80,000 to 150,000 g/mol. If the shell molecular weight is too low then the degree of grafting is considerably reduced. If the shell molecular weight is too high then the average gloss becomes too high.
Shell molecular weights can be controlled by various methods known in the art, the most preferred method is to use a chain transfer agent in the amounts of from 0.005 to 5.0, preferably from 0.05 to 2.0, and most preferably from 0.1 to 2.0 weight percent based on shell polymer weight during the shell polymerization. A chain transfer agent may be used to control the molecular weight of the shell polymer and is important for providing capstock compositions that are both processable and which have a reduced gloss appearance. If less than 0.005 weight percent chain transfer agent is used then the shell molecular weight becomes too high and the viscosity increases, thereby resulting in greater energy needed for processing. If the chain transfer agent amount is greater than 5.0 weight percent then the degree of grafting of shell polymer becomes too low resulting in degraded performance.
Other ways of reducing molecular weight during typical emulsion polymerization processing include: increasing the polymerization temperature and/or with a gradual feeding of the monomers; increasing the initiator level; feeding in the shell monomers neat versus preemulsifing the monomers; and improving the degree of mixing of the reaction medium.
Suitable polymers for the outer shell of the second core/shell polymer require that they have a glass transition temperature (xe2x80x9cTgxe2x80x9d) above 20xc2x0 C. and therefore may also contain one or more units derived from ethylenically unsaturated copolymerizable monomers which are different from the at least one C1-C8 alkyl methacrylate monomer.
Suitable ethylenically unsaturated copolymerizable monomers include one or more of any of the following monomers: C1-C8 alkyl (meth)acrylates, acrylonitrile, methacrylonitrile, divinyl benzene, alpha-methyl styrene, para-methyl styrene, chlorostyrene, vinyl toluene, dibromostyrene, tribromostyrene, vinyl naphthalene, isopropenyl naphthalene, as well as higher carbon (C9-C20) alkyl (meth)acrylates such as decyl acrylate lauryl methacrylate, lauryl acrylate, stearyl methacrylate, stearyl acrylate, isobornyl methacrylate, and the like. The C1-C8 alkyl (meth)acrylate monomers are preferred for their enhanced weatherability characteristics. Most preferred are the C1-C8 alkyl acrylate monomers.
One or more chain transfer agents can be used to control the molecular weight of the shell polymer of the second xe2x80x9chigh rubberxe2x80x9d core/shell polymer. Common chain transfer agents or mixtures thereof known in the art include the C4-C18 alkyl mercaptans, mercapto-group-containing acids, thiophenols, carbon tetrabromide, carbon tetrachloride, and the like. They may be used alone or as mixtures thereof. Examples of the C4-C18 alkyl mercaptans include butyl mercaptan, hexyl mercaptan, n-octyl mercaptan, decyl mercaptan, lauryl mercaptan. t-dodecyl mercaptan and n-dodecyl mercaptan are preferred.
The invention also encompasses having other or additional stages, which are polymerized after the formation of the rubbery core stage. Such stages may include an additional rubbery stage of a poly(alkyl acrylate), or additional outer stages of polymer mainly or exclusively polymerized from C1-C4 alkyl methacrylate. Other multi-staged core/shell polymers are envisioned, i.e., hard-core/rubbery shell/hard shell polymers.
In preferred form, the first xe2x80x9cmedium rubberxe2x80x9d core/shell polymer is an acrylic resin prepared according to the free-radical emulsion polymerization procedure described in U.S Pat. No. 3,812,205, which is incorporated herein by reference for its disclosure on the preparation of acrylic core-shell polymeric materials prepared with graftlinkers using emulsion polymerization techniques. The second xe2x80x9chigh rubberxe2x80x9d acrylate-based core/shell polymer is also prepared by a free-radical polymerization as described in EP-A-850,740, which is incorporated by reference for its disclosure on the preparation of high rubber acrylic impact modifiers made using emulsion polymerization.
The core-shell polymers may be isolated from the emulsion in various ways, the preferred methods being spray-drying or coagulation, such as with electrolyte addition. Any of the various techniques described in the literature, such as U.S. Pat. No. 4,897,462, may also be applied to the emulsion during isolation to produce a spheroidal product which, when dried, exhibits outstanding powder flow, low dusting, and higher bulk density than conventionally isolated powders.
The capstock composition may further contain from 0 to 5, preferably from 0.5 to 3, most preferably from 1 to 2 parts by weight of at least one UV light stabilizer. Many suitable UV light stabilizers are described in xe2x80x9cPlastics Additives and Modifiers Handbook, Ch. 16 Environmental Protective Agentsxe2x80x9d, J. Edenbaum, Ed.., Van Nostrand (1992) pp. 208-271, which is incorporated herein by reference for its disclosure of UV light stabilizers. Preferred UV light stabilizers are of the HALS-, benzotriazole-, and benzophenone-type compounds. These compounds further enhance the weatherability of the capstock composition. Many such compounds are commercially available from Ciba Specialty Chemicals (Tarrytown, N.Y.) under the TINUVIN tradename.
The capstock composition may further contain from 0 to 100 parts by weight of at least one polyvinyl chloride resin (xe2x80x9cPVCxe2x80x9d). Because total parts by weight in the capstock composition do not necessarily add to 100, the addition of a maximum of 100 parts by weight PVC to the capstock composition results in a weight ratio of PVC to first and second core/shell polymers of 100:100, or about 50 weight percent. The addition of other components follows this weight fraction protocol. Although the addition of PVC has a tendency to reduce the gloss of the capstock, it also has the effect of reducing the ability of the capstock to withstand weathering.
The capstock composition may further contain from 0 to 20 parts by weight of at least one pigment. Many suitable pigments are described in xe2x80x9cPlastics Additives and Modifiers Handbook, Section VIII, xe2x80x9cColorantsxe2x80x9d, J. Edenbaum, Ed., Van Nostrand (1992), pp. 884-954 which is incorporated herein by reference for its disclosure of various pigments useful for coloring plastics. Examples include organic pigments and inorganic pigments, and those preferred are resistant to UV and visible light exposure such as titanium dioxide (white), clays (beige) and slate blue pigment (blue).
The capstock composition may further contain from 0 to 15 parts by weight of at least one matting agent. Suitable matting agents include relatively large organic and inorganic particles (e.g., glass, ceramic and polymer) which are used for altering the light scattering properties in plastic resins. Numerous inorganic and polymeric matting agents for acrylic resins are described in U.S. Pat. No. 5,346,954 which is incorporated by reference for its disclosure of matting agents useful for blending with acrylic resins.
The capstock composition may further contain from 0 to 5 parts by weight of a powder flow aid. Suitable powder flow aids may be incorporated in the spray drying process used for recovering dry powder capstock composition. An example is stearic acid-coated calcium carbonate. Flow aids are further described in U.S. Pat. No. 4,278,576 which is incorporated by reference for its disclosure of flow aids useful for spray drying emulsions of core/shell polymers.
The process which has been found for preparing a capstock capable of providing the requisite impact strength, high color retention, and reduced gloss comprises the steps of first preparing a mixture (I) having a capstock composition according to the first aspect of the present invention. As described above, dry powders of the first and second core/shell polymers can be prepared by recovering emulsion core/shell polymers either by spray drying or by coagulation followed by wet-cake drying. These core/shell polymers may be recovered separately as individual powders which are subsequently mixed together using a suitable powder mixing device (i.e., ribbon blender) to prepare a dry powder mixture. Alternatively, the first and second core/shell polymers may be blended in the emulsion state and subsequently recovered as a mixed dry powder blend by either co-spray drying or coagulation followed by drying.
Additional components in the capstock composition, such as UV stabilizers, pigments, PVC resin, matting agents, flow aids, processing aids, lubricants, fillers, and the like, may be blended in either powder or liquid form with the first and second core/shell polymers that comprise the base resin capstock composition. Individual additives, i.e., UV light stabilizer, may be emulsified, added to the core/shell emulsion polymers and co-spray-dried. Alternatively, emulsified additives, such as pigment dispersion may be added directly to core/shell polymer powder in a suitable mixing device which allows for the addition of heat and the removal of water. Likewise, PVC wetcake may also be blended with powder or aqueous-based core/shell polymers. Numerous combinations of mixing emulsion-based additives and powders followed by subsequent drying can be envisioned by one skilled in the art.
If a pelletized form of the capstock composition is preferred for preparing capstock film, sheet, and other various articles instead of a powder (e.g., to avoid dust), then the powder may be formed into pellets using any suitable plastics pelletization equipment and methods known in the plastics processing art. This can be especially useful in combination with the mixing step wherein the components of the capstock composition can be compounded (mixed) and pelletized using standard plastics processing equipment.
In step (II) the mixture (I) is fed into a plastics processing device, such as an extruder, which is well known to the plastics-processing art. Typically, an extruder having a feed section and a metering section is utilized. Further details can be found in Principles of Polymer Processing, by Z. Tadmor and C. G. Gogos, John Wiley, 1979.
An ideal set of extruder operating temperatures exists at which the capstock composition can be processed efficiently to create a reduced gloss appearance while maintaining high impact strength. The feed section temperature of the extruder is adjusted so that the pellets or powder of mixture (I) do not melt and stick in the feed throat In processing the mixture (I) there are sometimes problems with feeding the powder form into an extruder, which is exacerbated by relatively fast fusion (melting) of the mixture (I) causing the molten mixture to back up into the feed throat. Fine powder forms of (I) makes feeding onto the screw flights of the metering section difficult, particularly with smaller extruders which have shallow flight screws. Reducing the feed section temperature partially alleviates this problem and output is improved, but the gloss, though reduced by the lower processing/melt temperature can be too high. Thus, even lower processing temperatures are needed to ensure that the gloss is low enough but this causes a low throughput problem. In order to increase throughput, the fusion of (I) should be delayed.
Optionally, addition of relatively small amount of an external lubricant, such as an oxidized polyethylene wax, helps to increase the fusion time and to reduce clogging of partially fused polymer in the feed throat. The addition of an external lubricant therefore helps to provide a higher output.
It is important that during the step (III) of metering and melting the mixture (I) in the metering section to form a melt, the temperature of the metering section must be kept between 165xc2x0 C. and 190xc2x0 C., preferably between 170xc2x0 C. and 185xc2x0 C., and most preferably between 175xc2x0 C. and 180xc2x0 C. If the metering section temperature is too low then the capstock will have poor impact; higher temperatures can help improve impact strength, but too high temperatures can lead to higher gloss.
The melt temperature must be kept between 165xc2x0 C. and 195xc2x0 C., preferably between 170xc2x0 C. and 190xc2x0 C., and most preferably between 170xc2x0 C. and 188xc2x0 C. If the temperature is above 195xc2x0 C. then the gloss increases to an unacceptable level. However, if the melt temperatures is too low then the viscosity of the resin becomes too great and the flowrate through the plastics processing device must be reduced to avoid viscous heating which deleteriously reduces process efficiency.
The step (IV) of forming the melt into a melt layer in a die located at the end of the extruder is done within a suitable plastics forming device, such as a die, as is known in the art (See Id., Ch. 13, xe2x80x9cDie Formingxe2x80x9d). For preparing capstock it is best to form the melt into a thickness of from 0.1 to 1.0 mm thick, which is useful as protective layers for PVC building products (e.g., PVC siding, window frames, fencing, decking, and rain gutters).
The steps (V) of extruding the melt layer from the die and (VI) of cooling the melt layer after it exits the die are also known plastics processing steps. Cooling the extruded melt layer can occur by passing the melt layer through a cooling fluid medium such as a liquid (i.e., water) or a gas (i.e., air) having a temperature sufficiently low to cause the capstock to harden. The temperature of the cooling fluid should be kept below the hardening temperature, i.e. Tg, of the polymeric component having the highest Tg in the composition. As an example, capstock compositions including core/shell polymers having PMMA shells of a Tg of about 100xc2x0 C. and require a cooling fluid, i.e., water, having a temperature of about 80xc2x0 C. or less.
Alternatively from, or in addition to using a cooling fluid, the melt layer can be passed and/or pressed between chilled rollers which may be polished smooth and/or have an embossing pattern. It is particularly preferable for capstock used for PVC siding applications to have rollers that provides an embossing pattern that produces a wood-grain effect into the capstock. Other embossing patterns are also envisioned for the chiller rollers, such as a matte finish. Such wood grain effect and matte-finish embossing patterns also tend to further reduce the gloss of the capstock and are therefore particularly desirable for use in the cooling step (VI) of preparing reduced-gloss weatherable impact-resistant capstock.
A method for making a synthetic resin composite is also envisioned which involves extruding a plurality of thermoplastic extrusion compounds and applying them together in a particular fashion. At least one of the thermoplastic extrusion compounds will be the capstock composition according to the first or second aspects of the present invention and disposed upon at least one other thermoplastic extrusion compound functioning as at least one substrate layer. It is also envisioned that the capstock composition can be extruded in multiple layers to allow for additional protection on one or more sides of the composite.
A typical capstock can be from 0.1 to 1.0 mm thick, whereas the structural plastic can be about 0.8 to 1.2 mm thick for PVC siding applications, and from 1.2 to 3.0 mm for PVC profile applications (e.g., PVC window frames, fencing, decking, and rain gutters). If the capstock and substrate are too thick then the articles made therefrom will suffer too great cost, whereas if they are too thin then they will be lacking in strength.
The substrate layer may also be formed by an extrusion of a thermoplastic resin. The thermoplastic resin may be any of the extrudable thermoplastic resins known in the art, examples of which are described in U.S. Pat. No. 5,318,737, incorporated herein by reference for its disclosure of extrudable resins and extrusion processes.
Preferred extrudable thermoplastic resins which are especially useful for making building products, but which require protection from a capstock layer against weathering and physical impacts, include PVC, chlorinated polyvinylchloride (xe2x80x9cCPVCxe2x80x9d), high impact polystyrene (xe2x80x9cHIPSxe2x80x9d), polypropylene (xe2x80x9cPPxe2x80x9d) and acrylonitrile-butadiene-styrene (xe2x80x9cABSxe2x80x9d). It is also preferred that the extrudable thermoplastic resins of the capstock and substrate layers adhere to one another to prevent delamination of the composite. Adhesion can be promoted through selection of resins which are compatible and/or miscible with one another (e.g., polymethyl methacrlyate-based resins and chlorinated resins). Various methods known in the art, such as surface treatment with adhesion promoters (i.e., corona discharge) and/or application of an adhesive, are envisioned for improving the adhesion between the substrate and capstock layers of the composite.
Synthetic resin composites can have a substrate layer of an extrudable thermoplastic resin, and a capstock layer of the capstock composition according to the first aspect of the present invention disposed thereon. The composites can be formed for example, by laminating preformed sheets or films of PVC structural plastic and the capstock together by thermal fusion or by adhesive.
Preferred extrudable thermoplastic resins used as the substrate layer include PVC, CPVC, HIPS, PP and ABS. Preferably, the capstock layer has an average gloss measured at a 75 degree incident angle geometry of less than 60, preferably less than 50, and most preferably below 45. Also, the capstock layer is preferred to have a drop dart impact strength of greater than 25 in-lbs at 23xc2x0 C. according to D4226. It is also preferred that the capstock layer has a xcex94E value of 8.0 or less and a xcex94L value of 2.0 or less after 1000 hours of accelerated weathering according to ASTM D4329 Cycle C.